Agree with Matt - if you can do the jumbie party at Leverick, that would be great. Fun for kids & adults!
I personally think the dinghy holds 6 adults comfortably. Better to make 2 trips than risk losing someone overboard.
Orion is well equipped and an all around great boat. Galley is well stocked for any cooking you'll do on board. There's a great "bag" attached to the aft cockpit for storing snorkel gear. It's going to be a bit tight with 10 people - put kids or smaller adults in the forward cabins. Assume you're using the single berth and the salon convertible for the kids? We used the empty single berth for our suitcases. Pack in duffle bags that fold down flat for storage.
You'll appreciate the shoreside AC on Orion when you're in the marina at TMM :-)